The cultivation of productive food crops is a major public health issue, as good nutrition is the foundation for promoting human fitness. Yet, reliance on chemical fertilizers is not sustainable and subsequent environmental contamination from their use threatens human health. An alternative strategy for generating fertile crops is to exploit plant growth-promoting bacteria (PGPB) that exist within diverse microbial communities in soil adjacent to plant roots (rhizosphere) and inside the root endophytic compartment (EC). In return for plant-derived carbon sources, PGPB increase plant biomass through beneficial immune stimulation and increasing bioavailable nutrients to the plant. Specifically, PGPB are a valuable source of nitrogen, the limiting nutrient in most natural soils and the main component of chemical fertilizers. Unfortunately, current efforts to inoculate field crops with PGPB often fail, likely due to competition with native soil microbial communities and limited EC colonization efficiency. Thus, there is a demand to study EC colonization by PGPB both in the context of the root microbiome community and changing nitrogen conditions. Previous work in the sponsor's lab used 16S ribotyping to define the root microbiome of the model plant Arabidopsis thaliana grown in natural soils. The microbial composition of the root EC was both taxonomically distinct from and less diverse than that of bulk soil, suggesting there are plant- and/or bacterial-derived factors governing root EC community assemblage. Under low nitrogen stress, plants modulate activity of metabolic pathways involved in nitrogen assimilation and change the composition of root exudates released into the rhizosphere. Combined with global changes in root architecture, these nitrogen status responses likely influence rhizosphere community structure. The goal of this project is to determine how the structure of the root microbiome changes in response to nitrogen stress, and how these community-level changes influence beneficial activities of individual PGPB within the community. By combining bacterial genetics, biochemical assays, and comparative genomics, this project will also help define core EC colonization and PGP traits required for improving plant growth in low nitrogen conditions. Results from these efforts will contribute to our understanding of the dynamics of natural rhizosphere communities that directly impact the health of field crops. Importantly, results from this project will also aid in the development of novel PGPB-based strategies that can compete in natural microbial soil communities to increase crop performance in an environmental friendly matter, directly improving human health through reduced usage of harmful fertilizers and improved nutrition.